In a radio communication system such as LTE (Long Term Evolution), an MIMO (Multi Input Multi Output) system is standardized (e.g. Non-patent Literature 1).
In the MIMO system, a base station apparatus maps a transmitting data string to a plurality of antennas and transmits the data. The transmitting data string is received by a terminal apparatus via the same number of propagation paths as (the number of transmitting antennas×the number of receiving antennas). In this case, the base station apparatus maps the data using a pre-coding matrix.
FIG. 15 illustrates an example of a pre-coding matrix. In a pre-coding matrix, the number of rows indicates the number of transmitting antennas, and the number of columns indicates the number of transmitting data (also called “streams”). The example in FIG. 15 indicates that two transmitting data strings are mapped to four transmitting antennas and transmitted.
A pre-coding matrix used for a base station apparatus and a terminal apparatus is limited to one in a code book. FIG. 16 and FIG. 17 are examples of a code book. FIG. 16 is an example of a code book in the case of two transmitting antennas, and FIG. 17 is the case of four transmitting antennas. In FIG. 16 and FIG. 17, “Codebook Index” indicates a code book number, and “Number of layers v” indicates number of transmitting data strings. The terminal apparatus selects a pre-coding matrix corresponding to the number of transmitting antennas and the number of transmitting data strings, and transmits a “Codebook Index” corresponding to the selected pre-coding matrix to the base station as a PMI (Pre-coding Matrix Indicator).
It is preferable that the pre-coding matrix is selected from the code book so that better throughput characteristics can be obtained on the propagation path. Therefore the terminal apparatus measures the propagation path (channel estimation), and selects a pre-coding matrix based on the result. The base station apparatus determines a pre-coding matrix based on the PMI, and maps the data string to each transmitting antenna, and transmits the data.
On the other hand, the number of streams of the transmitting data transmitted from the base station apparatus can be adaptively changed according to the environment of the propagation path. This technique is called “rank adaptation”, and is used for such a system as LTE (e.g. see Non-patent Document 1). Non-patent Document 1: 3GPPTS 36.211 V8.4